The Access Excellence Periodic Tableau

Acting Potential

This activity was made possible by Grant Number 8 R25 RR/AA09832-03 from the National Institutes of Health. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Institutes of Health; the grant recipient, the National Association of Biology Teachers; or the subcontractor, the Society for Neuroscience.

Thomas J. Conley
Parkway West High School
Ballwin, MO


Type of entry:

Lesson/class activity

Type of activity:

Hands-on, simulation

Target audience:

General Biology-adaptable to all levels

Background information:

Objectives

The students should be able to

Integration into curriculum

Class time needed

Materials Needed

Materials:

All of this can be purchased at a craft store. The sizes are not an absolute but it is important for the negative intracellular molecules to be the largest and for the two ions not to be able to move through the wrong channels. Different sizes and shapes for the Na+ and K+ ions were used to accomplish this. One could also use different sizes of dowel rods as indicated.

For the membrane use a 4 cm wide strip of foam board long enough to fit the length of your box. The pores for the Na+ and K+ need to be of appropriate size for the beads used. The pump was made by cutting a 25 mm circle from the foam board and by cutting out two notches on opposite sides. One notch has the shape and 1/2 the diameter of Na+ and the other notch was cut to fit the K+ ion. Make the following cuts: Na+ pore, K+ pore, pump site, Na+ pore, K+ pore, pump site. Put a small dowel rod through the center of the disc. The cut out in the membrane for the pump must be large enough to fit the carrier protein and the two molecules it carries. Using the beads from this setup, the opening was 21/2 X 11/2 inches. For the gates use the piece of foam board cut out when you made the opening in the strip and tape it to the membrane.See the following diagram which is not drawn to a set scale but is meant to give one a visual of these descriptions:

Notes for teacher

Students will work through all three activities. Do not stress the use of terms until the students have worked through each activity.


Activity 1
Exploration

  1. You will need to set up the cell membrane as described in background but do not let the students worry about the sodium-potassium pump portions for now. The gates for Na+ and K+ should be completely opened

  2. When you add the sodium and potassium ions, make sure there are more sodiums on the outside of the membrane and more potassiums on the inside. A few sodiums maybe placed on the inside of the membrane or a few potassiums on the outside.

  3. This activity is designed for the students to interact with each other. Let them mentally and orally interact and writing can be done at the end of each activity.

  4. When the students have arranged the beads properly, they will work through the four questions on their handout.

    Concept/Term Introduction

  5. Make sure they understand that diffusion is occurring and the idea that cells can perform functions that are needed for their survival even when the task goes against the moleculesı own forces.


Activity 2
Exploration

  1. For this activity the sodium-potassium pump will be used and place the negatively charged particles on the inside portion of the membrane. Put out more negatively charged particles than there are positively charged particles on the inside.

  2. Have the gates closed for sodium but partially opened for potassium.

  3. Do not tell them that the model is representing the axon of a neuron.

  4. Let them work out any possibilities as they do questions 1-3.

  5. If in their decision making no one has considered the role of the large negative molecules, then you will need to remind them of the attraction of opposites. Similarly, if they do not notice that some ions could leak through, you can add this variable.

    Concept/Term Introduction

  6. When they have looked at all of their possibilities and have given an answer to question 4 then you can mention that this represents the axon of a neuron and that it wants to maintain this imbalance so that the cell is positive on the outside and negative on the inside with a small intracellular negative voltage. This disequilibrium is maintained by the sodium-potassium pump.


Activity 3
Concept/Term Introduction

  1. At this point you will need to work with them as they read through the description on their handout. If your textbook has a better description, use it.

  2. Start with the first sodium gate opening and continue down the axon. It is important that they see this is a self-perpetuating system. As the first sodiums flow in, this triggers the full opening of the potassium channels and the opening of the next sodium channels.

  3. Also after the impulse passes one set of sodium channels, the impulse cannot go backwards because there is a brief period in which previous channels cannot be opened regardless of the voltage applied.

  4. Third, there is not just one channel opening at a time but many channels in the same area are being stimulated simultaneously by the change in voltage.

  5. Finally, the impulse is both a flow of electrical charge and a flow of molecules across the membrane. Application

  6. As the sodium moves in and changes the voltage of this area of the axon, the changed voltage also stimulates the next group of sodium channels. Thus the impulse is self-perpetuating.

  7. Impulse cannot flow backwards once initiated at the beginning of the axon because of the refractory period that results as the Na+ channels close and the K+ channels are activated.

  8. Since the stimulus is in the center of the axon and no refractory condition exists, the stimulus should cause the opening of Na+ channels on both sides of the stimulus. The result of this would be the flowing of one impulse towards the axonıs terminal end and one impulse moving towards the cell body and dendrite.

  9. Since the amplitude of the potential cannot increase regardless of the stimulus (remember the all or none principle), the increase in sensation is due to more frequent impulses being sent down the axon.

Fingertip Information